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Re: [PATCH 0/3] Introduce vhost-vdpa block device


From: Stefan Hajnoczi
Subject: Re: [PATCH 0/3] Introduce vhost-vdpa block device
Date: Mon, 26 Apr 2021 16:33:53 +0100

On Thu, Apr 08, 2021 at 06:12:49PM +0800, Xie Yongji wrote:
> Since we already have some ways to emulate vDPA block device
> in kernel[1] or userspace[2]. This series tries to introduce a
> new vhost-vdpa block device for that. To use it, we can add
> something like:
> 
> qemu-system-x86_64 \
>     -device vhost-vdpa-blk-pci,vdpa-dev=/dev/vhost-vdpa-0

This device is similar to vhost-user-blk. QEMU does not see it as a
block device so storage migration, I/O throttling, image formats, etc
are not supported. Stefano Garzarella and I discussed how vdpa-blk
devices could integrate more deeply with QEMU's block layer. The QEMU
block layer could be enabled only when necessary and otherwise bypassed
for maximum performance.

This alternative approach is similar to how vhost-net is implemented in
QEMU. A BlockDriver would handle the vdpa-blk device and the regular
virtio-blk-pci device would still be present. The virtqueues could be
delegated to the vdpa-blk device in order to bypass the QEMU block
layer.

I wanted to mention this since it's likely that this kind of vdpa-blk
device implementation will be posted in the future and you might be
interested. It makes live migration with non-shared storage possible,
for example.

An issue with vhost-user-blk is that the ownership of qdev properties
and VIRTIO Configuration Space fields was not clearly defined. Some
things are handled by QEMU's vhost-user-blk code, some things are
handled by the vhost-user device backend, and some things are negotiated
between both entities. This patch series follows the existing
vhost-user-blk approach, which I think will show serious issues as the
device is more widely used and whenever virtio-blk or the implementation
is extended with new features. It is very hard to provide backwards
compatibility with the current approach where the ownership of qdev
properties and VIRTIO Configuration Space fields is ad-hoc and largely
undefined.

Since vDPA has VIRTIO Configuration Space APIs, I suggest that the
vhost-vDPA device controls the entire configuration space. QEMU should
simply forward accesses between the guest and vhost-vdpa.

Regarding qdev properties, it's a little trickier because QEMU needs to
do the emulated VIRTIO device setup (allocating virtqueues, setting
their sizes, etc). Can QEMU query this information from the vDPA device?
If not, which qdev properties are read-only and must match the
configuration of the vDPA device and which are read-write and can
control the vDPA device?

Stefan

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